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Evidence from Discrepancies Between GPS Velocities and Rigid-Plate GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 10, 10.1029/2001GL013391, 2002 Deformation of convergent plates: Evidence from discrepancies between GPS velocities and rigid-plate motions Youqing Yang and Mian Liu Department of Geological Sciences, University of Missouri, Columbia, MO, USA. Received 8 May 2001; revised 24 October 2001; accepted 14 November 2001; published 30 May 2002. [1] Systematic discrepancies between the Global Positioning at the plate boundary, causing the coastal region to move System (GPS) site velocities and those predicted by plate-motion 30–40 mm yrÀ1 eastward with respect to stable South America models across the Andes, the Himalayan-Tibetan plateau, and the [Leffler et al., 1997]. The GPS velocity drops gradually over the Taiwan orogen delineate diffuse plate-boundary deformation and 1000-km wide Andean orogen, indicating a crustal shortening significant intraplate deformation, even in the presumably rigid pattern significantly different from that reflected in geological record [Liu et al., 2000]. oceanic plates. The relationship between the GPS velocities and the [4] The deformation of these convergent plates can be better predictions of plate-motion models is illustrated using a simple illustrated by comparing the GPS site velocities with the predic- timescale-dependent mechanical model of plate convergence. A tions of plate-motion models (Figure 1b). The comparison is simple stiffness number (f), based on the difference between the within the same reference frame of a fixed stable South America. predictions of plate-motion models and the GPS velocity near plate For each GPS site the corresponding rigid-plate velocity was boundaries, provides a convenient measure of the ‘‘plate-like’’ calculated from the Nuvel-1A model [DeMets et al., 1994] because behavior of convergent plates and strain partitioning between of the large area spanned by the GPS sites. The Nuvel-1A intraplate and interplate deformation. INDEX TERMS: 8102 velocities are identical along the small circle around the Euler Tectonophysics: Continental contractional orogenic belts; 8158: pole for the Nazca-South America relative motion (Figure 1a), Evolution of the Earth: Plate motions—present and recent (3040) whereas along the EISL-BRAZ profile the Nuvel-1A velocities deviate <1% from that along the small circle. So the contrasts between the GPS and Nuvel-1A velocities in Figure 1a reflect 1. Introduction primarily the difference between active crustal deformation and the prediction of plate-motion models. The plate boundary is marked [2] The basic tenet of the plate tectonics theory is that tectonic by the sharp drop of GPS velocity at the trench and diffuse plates are rigid and crustal deformation is concentrated within deformation across the Andes. Note that at all four sites on the narrowly defined plate boundaries. Studies in the past decades have Nazca plate the GPS velocity is consistently lower than the proven this tenet to be essentially valid in most places, especially Nuvel-1A predictions. Because the Nuvel-1A model was based for oceanic plates. One of the most satisfying tests is the general on marine magnetic anomalies averaged over the past 3 Myr, the agreement between the Global Positioning System (GPS) velocities lower GPS velocities were explained as indications of a slowing of and the prediction of rigid-plate motion models at many parts of plate convergence [Norabuena et al., 1999]. This is best shown by the Earth’s surface [Larson et al., 1997; Stein, 1993]. However, it is the 14 mm yrÀ1 difference between the Nuvel-1A and GPS well known that deformation diffuses over broad regions at some velocities at EISL, a GPS site near the spreading center. However, convergent plate boundaries, where the GPS velocities usually the 6mmyrÀ1 drop of the GPS velocity from EISL to ISFE near show a systematical deviation from plate-model predictions the trench may represent intraplate shortening of the Nazca plate, [Larson et al., 1999; Leffler et al., 1997]. Here we use such because these two sites are predicted to have almost identical discrepancies across the Andes, the Himalayan-Tibetan plateau, eastward velocity in the Nuvel-1A model. Similar drop of GPS and the Taiwan orogen to illustrate diffuse plate boundary defor- velocity is found at sites IRCR and GALA but with larger errors. mation in these regions. We investigate the relationship between The corresponding strain rate is up to 7 Â 10À17 sÀ1, comparable instantaneous and long-term averaged plate motions using a simple to that of some continental deformation. Although data from the timescale-dependent mechanical model, and discuss strain parti- Nazca plate may be too sparse to firmly delineate intraplate tioning at plate boundaries using GPS data. deformation, we show below that similar patterns of deformation are observed in other convergent plates. 2. GPS Velocities vs. Plate-Motion Model 2.2. The Himalayan-Tibetan Plateau Predictions [5] The India-Eurasia collision and continued convergence in 2.1. The Andes the past 50–70 Myr have led to the formation of the Hima- [3] The Andean mountain belt has resulted from subduction of layan-Tibetan plateau. The collisional deformation diffused thou- the Nazca plate underneath the South America plate in the past sands of kilometers into the Eurasia plate [Tapponnier and 30 Myr [Isacks, 1988]. The present-day plate convergence and Molnar, 1979]. This is clear from recent GPS measurements the Andean crustal shortening are revealed by recent GPS meas- (Figure 2a). For comparison we converted the GPS velocities urements [Angermann et al., 1999; Leffler et al., 1997; Norabuena from three groups [Chen et al., 2000; Larson et al., 1999; Wang et al., 1999] (Figure 1a). Between site EISL in the Nazca plate and et al., 1999] into those relative to stable Eurasia using the No- sites in stable South America (UEPP and BRAZ) there is Net-Rotation Nuvel-1A (NNR-A) absolute plate motion model 66 mm yrÀ1 convergence. About half of the convergence is [Argus and Gordon, 1991]. Note the diffuse deformation over the consumed by sliding in the subduction zone and the rest is locked Tibetan plateau. [6] Figure 2b shows that the GPS velocities at the interior India (IISC) and Eurasia (IRKT) plates are almost identical to the Nuvel- À1 Copyright 2002 by the American Geophysical Union. 1A predictions. Between these two stations there is 50 mm yr 0094-8276/02/2001GL013391$05.00 N-S convergence, agreeing with the Nuvel-1A model. As the sites 110 - 1 110 - 2 YANG AND LIU: DEFORMATION OF CONVERGENT PLATES on westerly striking planes in central India [Singh et al., 1999] including the 26 Jan. 2001 Gujarat earthquake. 2.3. The Taiwan Orogen [8] The Taiwan orogen is another example of complicated plate boundary deformation. It has resulted from convergence between the Philippine Sea and the Eurasian plates during late Cenozoic. The active mountain building is shown by the recent GPS measure- ments [Yu et al., 1997] (Figure 3). We mapped the GPS velocities onto the Eurasia-fixed reference frame by using the GPS velocity at TAIW (TAPN) [Larson et al., 1999] with respect to stable Eurasia and adding this velocity vector to the rest GPS site. The area covered by the Taiwan GPS network is small (about 2° Â 3°) that a uniform translation may be justifies, and there is no evidence of significant rotation of the Taiwan islands with respect to stable Eurasia. Figure 3 compares the GPS velocities with the Nuvel-1A. At Lanhsu, an island on the Philippine Sea plate, the GPS velocity is almost identical to the Nuvel-1A value. However, as the sites approach the Longitude Valley Fault (LVF), the present-day plate Figure 1. (a) GPS sites velocities relative to stable South America (data are from Angermann et al. [1999] and Norabuena et al. [1998]). The dashed line is a small circle through EISL around the Euler pole for the Nasca-South America relative motion. Along the circle site velocities should have the same magnitude for rigid plates. The solid line shows the location of the profile in (b). (b) Comparison of the GPS velocities (black dots with error bars) with Nuvel-1A model predictions (open circles) along the EISL-BRAZ profile. All Andean GPS sites within 200 km of the profile were projected. The dashed lines fit the Nuvel-1A velocity along the small circle in (a), and the solid line fits the GPS velocities. Only the east velocity component is shown for clarity. approach the Main Boundary Fault (MBF), the discrepancy between the GPS and the Nuvel-1A velocities increases. Across the MBF, the GPS velocity drops from 44 mm yrÀ1 at BIRA at the northern rim of the India plate to 23 mm yrÀ1 at LHAS in southern Tibet, representing 40% of the total N-S contraction between IRKT and IISC. The average strain rate is 1.7 Â 10À15 sÀ1 across the Himalayan front. The rest of the convergence is distributed across the Tibetan plateau and a broad region in central Asia. The nearly linear GPS velocity change over the Tibetan plateau is similar to that over Central Andes (see Figure 1b). Some recent GPS measurements [Freymueller et al., 2000] indicate a more Figure 2. (a) GPS velocities across the India-Eurasia plate smooth velocity drop across both the Himalayas and the Tibetan boundary with respect to stable Eurasia. The dashed line is a small plateau than those shown in Figure 2b. circle around the India-Eurasia Euler pole, along the circle the [7] Besides the broad deformation within the Eurasia plate, Nuvel-1A velocity has the same magnitude. The solid line shows Figure 2b also indicates significant shortening within the India the location of the profile in (b). Nuvel-1A velocities along this plate, the rigid indenter. Between IISC and the northern rim of the profile deviate <2% from that on the small circle.
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